1. Field of the Invention
This invention relates to a process for producing heating elements of silicon carbide, and more particularly to an improved process for producing silicon carbide heating elements of high strength and low resistivity.
2. Description of the Prior Art
Silicon carbide heating elements have hitherto been used as heating elements for use in electric furnaces. Recently they have also been used as heating elements for various industrial apparatuses. For these uses, high strength, low electric resistivity, a low negative temperature coefficient of resistivity, and high resistance to oxidation at high temperatures are required.
The following procedures have been developed for the sintering of silicon carbide (SiC):
(1) Japanese Patent Application (OPI) No. 78609/75 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".) filed by General Electric Co. entitled "PROCESS FOR PRODUCING A HIGH DENSITY SILICON CARBIDE BODY", describes a process in which a mixture of SiC, a boron compound and a carbon material is sintered in an argon (Ar) atmosphere at 2,050.degree. C. for 60 minutes to obtain a sintered product having a density of 95% or more of the theoretical density. This sintered product, however, is not suitable for use as a heating element, since its specific resistivity is more than several .OMEGA.-cm, and moreover its specific resistivity at high temperature is extremely reduced.
Where such a mixture is sintered in nitrogen at 2,200.degree. C. or less, the density of the sintered product obtained is only about 80% of the theoretical density, resulting in the formation of a sintered product of low strength, even though its specific resistivity is 1.0 .OMEGA.-cm or less. On the other hand, if the mixture is sintered in nitrogen atmosphere at higher temperatures, the specific resistivity is as high as 10.sup.1 to 10.sup.6 .OMEGA.-cm and the stability of its specific resistivity is low, even though it is sintered up to about 90% of the theoretical density.
(2) Japanese Patent Application (OPI) No. 110499/77 filed by the Carborundum Co. discloses "FUEL IGNITER COMPRISING A NOVEL SILICON CARBIDE COMPOSITION AND PROCESS FOR PREPARING THE COMPOSITION" in which a powder containing 95% or more SiC is sintered to 2.5 g/cc by hot pressing, and N, P, As, Sb and Bi in a solid or gas phase are doped therein to provide a specific resistivity at room temperature of 0.66 .OMEGA.-cm. This sintered product is also not suitable for use as a heating element of the energy-saving type, since its specific resistivity at 1,350.degree. C. is 0.12 .OMEGA.-cm, that is, its temperature coefficient of resistivity is not good.
(3) According to Japanese Patent Application (OPI) No. 121810/78, "HIGH DENSITY THERMAL SHOCK RESISTANT SINTERED SILICON CARBIDE", filed by the Carborundum Co., SiC, 0.3 to 3.0% by weight based on the weight of SiC, of an additive selected from BN, BP, AlB.sub.2 and mixtures thereof, and 150 to 500% carbon based on the weight of additive, heated at from 1,900.degree. C. to 2,500.degree. C. to provide a sintered product having a density of 85% or more of the theoretical density. This sintered product, however, is substantially the same as the sintered product of Japanese Patent Application (OPI) No. 78609/75, which is sintered at atmospheric pressure. Moreover, even though a sintered product has a density of 95% or more of the theoretical density, it is somewhat inferior in resistance to oxidation to those to which B and B.sub.4 C are added.